Fixing up history test to pass properly on supported platforms.

tools/dom/templates/html/impl/impl_History.darttemplate

Index: tools/dom/templates/html/impl/impl_History.darttemplate
diff --git a/tools/dom/templates/dart2js_impl.darttemplate b/tools/dom/templates/html/impl/impl_History.darttemplate
similarity index 53%
copy from tools/dom/templates/dart2js_impl.darttemplate
copy to tools/dom/templates/html/impl/impl_History.darttemplate
index 35ea8d4f1c717e62e81287d9995e09dc8358a951..943d8314691b6179250989aa8d680a74f26b87ba 100644
--- a/tools/dom/templates/dart2js_impl.darttemplate
+++ b/tools/dom/templates/html/impl/impl_History.darttemplate
@@ -6,4 +6,19 @@ part of $LIBRARYNAME;
 
 /// @domName $DOMNAME; @docsEditable true$ANNOTATIONS
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
+
+  /**
+   * Checks if the State APIs are supported on the current platform.
+   *
+   * See also:
+   *
+   * * [pushState]
+   * * [replaceState]
+   * * [state]
+   */
+$if DART2JS
+  static bool get supportsState => JS('bool', '!!window.history.pushState');
+$else
+  static bool get supportsState => true;
+$endif
 $!MEMBERS}